Who?
Walter Dougherty, master of disguise and natty dresser. Which are kind of the same thing, in his case.

When?
From 1987 to the present, though it feels much longer.

What?

“Now he's somewhere in a circus scene! Henchmen, help me find him! Is that him with the jugglers? No? Damn! At this rate I'll never have time for my nefarious scheme. He's over by the elephants...no, wait. WAL-DOOO!”

Wally (or Waldo), as he was commonly known, was the preeminent spy on Her Majesty's Secret Service[1].

Waldo toured the world, playing backgammon or roulette while smoking furiously and drinking martinis, also furiously. In addition he was also known to have seeded many fertile fields, as it were.

Oh, and he saved the world.

A bunch of times.

Why?

“Man goes in the cage, cage goes in the water...shark’s in the water...”

~ Robert Shaw on getting his roles in From Russia With Love and Jaws confused

"We're gonna need a bigger boat."

Her Majesty, the Queen[2], needed a force of super spies to thwart the many excessively wealthy and quirky madmen who were planning to take over the world.
Madmen do stuff like that. That's what makes them "mad". They'd just be men otherwise, and therefore little or no threat at all.

She, with the aid of an obscure subdepartment[3] of the Ministry of Defence, the Office of Defending the World from Threats That Don't Exist, Except Within the Heads of People Who Really Should Know Better By Now. I Mean, Really! They Can't Get My Mail to Me on Time, but They've got Tax Dollars to Waste on This Foolishness? Bollucks! I'm of a Mind to Draft a 'Letter to the Editor' This Very Instant![4] put an ad in the local penny saver, seeking a

"...debonair and dashing spy of breeding and class for spying, poorly choreographed fisticuffs and saving the world. Typing and filing experience an asset. No fatties."

Waldo's disguise, on display at the Smithsonian in amongst one hundred or so other similar displays

Instead, they settled for Waldo.

Waldo, with his unique fashion sense, possessed an uncanny aptitude for blending into crowds, particularly when those crowds were dressed a manner that was similar, but not quite identical, to his own striped top and tuque. When the crowds were not so garishly dressed he tended to blend in...not so much, as his vacation photos in "Pennsylvania Dutch" country attest.

This camouflage related weakness led to his untimely demise at the hands of the very first villain that he encountered[6].

Luckily Waldos come in a book, so when one dies the Office of Defending the World from Threats That Don't Exist, Except Within the Heads of People Who Really Should Know Better By Now. I Mean, Really! They Can't Get My Mail to Me on Time, but They've got Tax Dollars to Waste on This Foolishness? Bollucks! I'm of a Mind to Draft a 'Letter to the Editor' This Very Instant![7] just have to flip over to the next page.

↑As in a smaller department within another department. That other department is larger because it needs to contain the smaller department within its boundaries, as it's considered unseemly in polite society to have subdepartment leaking out everywhere. Gross!

↑The villain wore a grey jacket with a nehru collar and grey pants. How was Waldo supposed to blend in with that? He wanted to go fight the villain with the striped sweater and big, round glasses in the scene at the beach, but noooo, the Queen is all, "Go fight Dr. No at his secret underground island base, which has no people with glasses, tuques or sweaters whatsoever."